1. In Records, click Constituents and open the current head of household's constituent record.
2. Click the Spouse button on the Bio 1 tab.
3. Mark the Head of Household checkbox on the General 1 tab of the spouse's relationship record. This checkbox is only available if the spouse has its own constituent record.

For more information on head of household processing, see: What is Head of Household processing and how to use it?